1. Field of the Invention
This invention generally relates to solar or light energy, and more specifically, the invention relates to photovoltaic cells.
2. Background Art
Solar energy has long been looked to as a significant part of a solution to the ever increasing energy needs of the planet's population. Increasing costs of mining fossil fuels and increased concerns over “greenhouse” emissions have furthered interest in exploiting alternative energy strategies, including solar energy sources. To date, solar energy conversion has generally relied upon either the direct harvesting of solar thermal energy, e.g., in heating applications, or in thermoelectric conversion, or through the direct conversion of photonic energy to electrical energy through the use of photovoltaic cells.
Photovoltaic cells have developed according to two distinct methods. The initial operational cells employed a matrix of single crystal silicon appropriately doped to produce a planar p-n junction. An intrinsic electric field established at the p-n junction produces a voltage by directing solar photon produced holes and free electrons in opposite directions. Despite good conversion efficiencies and long-term reliability, widespread energy collection using single-crystal silicon cells is thwarted by the exceptionally high cost of single crystal silicon material and interconnection processing.
A second approach to produce photovoltaic cells is by depositing thin photovoltaic semiconductor films on a supporting substrate. Material requirements are minimized and technologies can be proposed for mass production. The thin film structures can be designed according to doped homojunction technology such as that involving silicon films, or can employ heterojunction approaches such as those using CdTe or chalcopyrite materials. Despite significant improvements in individual cell conversion efficiencies for both single crystal and thin film approaches, photovoltaic energy collection has been generally restricted to applications having low power requirements.
In all solar cells, especially the high performance ones, series resistance issue is a serious limiting factor for cell performance. This series resistance is associated with the contact electrode geometry, emitter (top) layer sheet resistance, and metal-semiconductor contact resistance. The design of the electrode structure must consider the trade off between area coverage and the shadowing loss. A standard approach is to use comb-like metal grids or to use transparent conducting oxide (TCO) film that combines transparency and good conductivity of the film. However such TCO film still has a limited conductivity and still contributes to considerable amount of series resistance.